A set of standards have been promulgated by the Institute of Electrical and Electronic Engineers (IEEE) which specify the port characteristics for a coaxial media interface to Data Terminal Equipment (DTE). In an IEEE 802.3 (Type A)/Ethernet application, a coaxial (0.4 inch diameter) media interface is established to the DTE through an isolating pulse transformer. In an IEEE 802.3 (Type B)/Cheapernet application, the coaxial (0.2 inch diameter) media interface is established directly to the DTE.
At the coaxial media interface, the amplifier used for supplying data for transmision over the interface generates signals for a "tap driver" amplifier which must meet exacting output requirements. When transmitting, the tap driver provides an output current which is 41 milliamp (mA) on the average and 82 mA peak, with a square-wave minimum rise time and fall time of 20 nanosecond (nS) and maximum rise time and fall time of 30 nS. When not transmitting, the tap driver should exhibit an input resistance greater than 100 kilo ohms.
The prior art employed discrete components and implemented current switching through delay lines and post-switch filtering to approximate the output waveform. Alternatively, analog filtering of a square wave was used. The former approach is not suited for an integrated circuit because of the high power required to drive steered current and the high power dissipation associated with steering current from the load to the integrated circuit when transmitting.
A gated current could be generated which would eliminate the non-transmitting power dissipation, but would not result in lower power dissipation during transmission into a short circuit. Accordingly, the prior art amplifiers are unsuited for use in tap driver to be constructed on a single integrated circuit chip package capable of dissipating one watt total power.
A node of a network satisfying the IEEE 802.3 standards must also evidence a precise dc voltage when transmitting for detection by other network nodes, so as to avoid "collisions". A high-frequency ac signal is superimposed on this dc voltage; the ac signal representing the data transmitted over the network.